DESCRIPTION (Investigator's Abstract): Nitroglycerin is a vasodilator widely used in the treatment of patients with ischemic heart disease, however, the long term efficacy of nitroglycerin and related drugs is limited by the development of tolerance. The long term objective of this proposal is to clarify the cellular mechanisms underlying nitroglycerin-induced tolerance in coronary arteries. The overall working hypothesis is that nitroglycerin produces relaxation of coronary artery smooth muscle by at least two independent mechanisms, i.e. increased cyclic guanosine monophosphate (GMP) production and increased potassium ion conductance. Moreover, it is proposed that the cyclic GMP mediated mechanism is uniquely sensitive to the tolerance process, such that the effects of nitroglycerin become more dependent on potassium conductance in nitrate tolerant arteries. This hypothesis will be addressed by using an in vitro model of nitrate tolerance in isolated coronary arteries. Functional studies, in which isometric tension is recorded, will be correlated with biochemical measurements of cyclic GMP levels and with patch clamp recordings of potassium currents in single coronary smooth muscle cells. The effects of nitroglycerin and other nitrovasodilators will be studied in nitrate tolerant and nontolerant tissue. The role of increased cyclic GMP metabolism in nitrate tolerance will be assessed, as well as the ability of nitroglycerin to activate potassium currents in tolerant and nontolerant smooth muscle cells. The effects of nitrate tolerance on smooth muscle relaxation by both cyclic GMP-dependent and cyclic GMP-independent endothelium-derived relaxing factors will also be assessed. By determining the mechanisms underlying nitrate tolerance at the level of the smooth muscle, it is anticipated that this investigation will shed new light on the clinical problems associated with nitroglycerin therapy, and on the basic pharmacology and physiology of the nitrovasodilators. Moreover, the results obtained may suggest new strategies for the development of novel nitrogylcerin-like drugs that do not cause tolerance.